The invention relates generally to integrated optical elements, and more particularly to optical logic elements capable of being controlled to produce a variety of logic operations.
The advancing state of the art of integrated optics and the availability of small, relatively inexpensive integrated optical elements, makes optics an attractive alternative to certain processing operations previously performed using electronic circuitry. One reason for this attractiveness is that the large bandwidths available with optical devices permit high speed operation which is necessary for processing high data rate signals in real-time. Theoretically achievable speeds at optical frequencies are in the order of 1 to 10 picoseconds. This speed is significantly faster than that obtainable with electronic logic gates, whose ultimate switching speed will probably be limited by capacitance to about 300 picoseconds. Such potential speeds make optical logic very desirable for certain high speed signal processing applications. Other advantages of optical elements include their freedom from cross talk and intermodulation which are present in electronic devices, and their relative insensitivity to electromagnetic interference effects.
Optical devices which utilize electrical switching and are capable of performing fixed logic operations such as OR or AND have been demonstrated. Such devices may typically operate on the principle of constructive or destructive interference between two or more light beams, i.e., two light beams in-phase with each other which are added together reinforce to produce a light beam having an amplitude equal to the sum of the amplitudes, while two light beams of opposite phase cancel one another, the energy being redistributed to regions beyond the area of overlap such that the waveguide propagating this energy is inhibited. To perform a processing operation, it is typically necessary to interconnect a large number of different logic gates together, generally in a pipeline configuration, so that a specified sequence of particular logic operations is performed on the input signals. When fixed function logic gates are used, the processing operation is fixed and determined by the particular types and interconnection of the gates. To vary the processing operation, it is necessary to vary either the types of gates or their interconnection.
In some applications, it is desirable to be able to dynamically vary the processing operation. With fixed function logic gates, this requires that switching means be provided internal to the logic configuration so that signals can be routed to different gates. The flexibility of such a processor is, therefore, natually limited to the flexibility available in the switching means, the availability of gates to perform necessary logic operations and the ingenuity of the designer. In optical processors which depend upon the interference between light beams to perform logic operations, the necessity of precisely controlling the phase of the optical signals input to the logic gates presents an added problem in achieving flexibility, where the light beams must be capable of being switched to a plurality of different locations in the optical logic.
In addition to the added hardware required for providing switching between logic gates, a further disadvantage of using fixed function logic gates where flexibility is desired, is the necessity of providing a large number of the different types of gates which may be required to achieve the desired degree of flexibility. This increases the size and cost of the processor, adds to its complexity and results in inefficient utilization of hardware. These problems can be offset to some extent by utilizing logic gates which are capable of performing more than one logic function, thus reducing the number of logic gates required.
An optical logic element capable of performing several optical operations simultaneously is disclosed in U.S. Pat. No. 4,128,300 to Stotts et al. The optical devices therein disclosed provide several different outputs from the optical element, which represent different logic operations, e.g., AND, EXCLUSIVE OR, NOR, etc., and simultaneously functions as several different logic gates. The optical devices operate by coupling to different optical propagating modes in a multi-mode optical waveguide. The various logic functions are obtained by coupling to different optical modes, using output waveguides which are appropriately sized to propagate the specific mode desired. Each separate logic function requires a separate output from the logic element. In essence, the devices disclosed are mode selective switches. By launching particular modes in the multi-mode waveguide, outputs will be obtained from the appropriate waveguides capable of coupling to the various modes.
One of the difficulties with the devices disclosed in the aforementioned patent, is that in order to interconnect a number of such logic elements together, means must be provided for converting between high-order modes output from one device and the low-order modes required for input into subsequent logic devices. Furthermore, the devices disclosed do not permit dynamic processing, since the logic operations performed by the gates can not be varied, except through the use of complicated switching schemes, as previously described.
A further and more important disadvantage of previous optical logic gates is that they depend upon the switching of electrical control voltages, in coincidence with the presence or absence of inputs to the gates, in order to perform a logic operation. This is necessary in order to change the relative optical phases in the appropriate waveguides to obtain the logic function. Thus, in prior optical logic gates, the speed at which a given logic operation can be performed is limited not by the inherent optical bandwidth, but rather by the electrical switching speed, which is determined primarily by the control electrode capacitance.
The present invention does not require control electrode voltage switching to perform logic operations. Logic operations result from the constructive and destructive interference between light beams which propagate through the optical logic element. Control electrodes are utilized only to "configure" the logic element to perform a given logic function, i.e., AND, OR, etc., by adjusting the optical lengths of predetermined paths to a "fixed" length. Once the logic element is configured for a particular logic function by the application voltages to certain control electrodes, the voltages remain fixed until it is desired to change the logic function. Therefore, the rate at which logic operations can be performed is determined by the optical bandwidth of the optical device, not the speed at which the control electrodes can be switched.